In the conventional wireless access system, user terminals are connected to a wired network as represented by the wireless LAN via access points in order to enjoy the Internet services. In the existing wireless LAN, interference may be avoided through time multiplexing of the communication time between a plurality of user terminals and access points with a structure of carrier sense provided to detect communication devices which are making communication using the common frequency band but here rises a problem that since amount of data which can be transmitted in the unit time does not increase, the throughput of the entire system does not increase even when the number of access points is increased and thereby the throughput of an access point is lowered.
The JP-B No. 180775/2002 (U.S. patent Ser. No. 10/449,100) filed previously provides a structure to increase the throughput of the entire system by simultaneously making communication among a plurality of access points as a measure to solve the problem described above.
FIG. 1 is an explanatory diagram of the method for improving the throughput with parallel transmissions among access points. In this figure, the access points 101a, 101b, and 101c are simultaneously making communications with user terminals 102a, 102b, and 102c. In this case, the simultaneous communications are realized by controlling the service area of the access point 101a so that the radio wave of the data transmitted from the access point 101a is not detected with a carrier sense of the access point 101b. The radio wave is not detected with a carrier sense under the condition that the power of the radio wave transmitted from the access point 101a goes lower than the reference received power preset in the access point 101b (hereinafter, referred to as a carrier sense threshold).
Similarly, the JP-B No. 180775/2002 provides a structure to control the service areas among the access points for parallel transmissions so that the radio wave transmitted from the access point 101b is not detected with a carrier sense of the access point 101c. 
In the wireless access system described above, a structure to control the operations in order to satisfy the condition to simultaneously transmit data communication packets is described but this wireless access system does not refer to a problem that the packets transmitted are not correctly received because of interference of the other data communications and are sometimes discarded.
FIG. 2 is an explanatory diagram of signal pair interference power ratio in the wireless access system. The access point 101a transmits data packet in the transmission power AP1_TXP and the user terminal 102a receives the desired signal in the received power AP1_STA1_RXP. Moreover, the user terminal 102a receives the signal, which has been transmitted from the access point 102a as the data packet to the user terminal 102b in the transmission power AP2_TXP, as the interference signal in the received power AP2_STA1_RXP. In this timing, a power ratio of the desired signal and the interference signal becomes equal to the signal pair interference power ratio. Although, depending on the demodulation performance of a receiver, when the signal pair interference power ratio SIR is equal to or higher than the desired signal pair interference power ratio SIR_req, the data signal is demodulated correctly, but if it is equal to or lower than the desired signal pair interference power ratio SIR_req, the data signal is demodulated erroneously due to the collision of packets.
FIG. 3 is a graph showing an example of geographical distribution of the signal pair interference power ratio SIR. This figure indicates amplitudes of signal pair interference power ratios (SIRs) when the receiving places are changed in the data communication performed between both access points with the same radio-communication channel in the equal transmission power in the case where a room of 10 m square is assumed and the access point 101a and access point 101b are provided to the corners of this room. When the desired signal pair interference power ratio SIR_req is assumed as 10 dB, the predetermined signal pair interference power ratio SIR_req may be satisfied within the range of 4 m around the access points even when the communications are simultaneously conducted by abovementioned two access points. The domain in which the predetermined signal pair interference power ratio SIR_req is satisfied is hereinafter referred to as the non-interference domain. In the domain isolated by 4 m or more away from the access point, the predetermined signal pair interference power ratio SIR_req cannot be satisfied. This domain is hereinafter referred to as the interference domain. When two access points described above make communications simultaneously to the user terminals in this interference domain, here rises a problem that data is lost due to the collision of packets.